Are you flying or just renting a car to use? If it is really remote and you have no source of electricity I would look into taking a battery (high amp deep cycle) and a solar battery charger. I wouldn't tempt fate with the battery on the car if you are really remote.

If you are flying it might be worth stopping by the auto store on your way out of town. Then either ship the battery of give it to a local astronomer

It doesn't take all that long to deplete a car battery, a trunk light left on for 6-8 hours will do it in..... so be minimal about using the car battery to run stuff.
I have charged a spare car battery from a running car by hooking it up with jumper cables, and nothing was damaged, but it took a while, because an idling car doesn't spin the alternator fast enough to put out full voltage, and I've seen someone with a cigarette lighter plug and wires with clips on the other end, that he uses to keep a spare battery that sits on a tray on the passenger floor charged up that he uses in his camper for lighting, but that charges while he's driving. Be aware that a charging battery gives off smelly, flammable gas (though not a lot).

If it is going to be sunny where you're going, there are fairly inexpensive solar panel chargers that you could top your portable battery up each day with.

Also, other members might write with reasons why my running auto ideas are dangerous, so wait a bit before trying.....

I am planning a one week trip to a remote area, where my only source of power will be the battery of my rental car.

1. What do I need to prevent that I deplete the battery to a level where it cannot start the engine? Where do I get such a gadget?

2. Suppose that I buy a second battery. What do I need to recharge it in my car? And also to prevent that the charging of the second battery depletes the car battery. Where can I buy such a device?

Thank you,
--Dom

The extra battery sounds like a good idea . Go for a deep cycle (marine battery) . A good pair of jumper cables will work for charging but not practical if you want to drive while charging . A cigarette plug to the extra battery on the passenger side floorboard would allow a charge while driving but the hazard of a lead acid battery in the cab is probably not a good idea .
There is this thing called a battery isolator whereas two batteries charge while driving but only the accessory battery will discharge for accessories saving the the start battery for the vehicle . This is more or less a permanent installation on the vehicle being hard wired in , so on a rental car it doesn't really fill the bill .
Why not a small generator ? Harbor Freight has a little 1500 or 1000 watt unit that they have been selling for like 98 bucks . Then again a small solar panel array could recharge if it's sunny in the day time .
A power chair battery is an AGM type . (Absorbed Glass Mat) they are maintenance free and can't spill . They are also deep cycle , so that would really be the ticket . Then charging on the floorboard while driving would be acceptable . Check the bay for these . How many amp hours a night would you need ? 35ah , 70ah , 100ah ? There are all sizes available on the various web sites . I'd say 100 to 200 bucks should get you into the ball game depending on how big you go . Then just a cigarette lighter plug and a set of clips to the battery and away ya go .
That is if the rental car has a cigarette lighter socket . Do they even put cigarette lighters in cars anymore ? My newest vehicle is twenty years old so .......

Afew years ago (before cell phones were common) several club members had an observing session at a remote site. It was a nice night but eventually we all left for home, except for one guy. He was working on his Herschel list and didn't want to pass on the opportunity. His opportunity included walking back to civilization when his scope drive depleted his car battery.

If you do use the car as power source, I would start and run the engine for about 10 minutes every 45 minutes or so. This should keep the battery from draining to a point where the car won't start. Otherwise I would recommend get a portable battery to run your equipment. Barry

Buy one or even two of those "power tanks." They can be recharged by plugging them into a 120-volt outlet, and there are some that have a 17 Amp-hour capacity. If you are going to stay in a nearby hotel or other establishment nearby, that would be the route to take if there's no AC power there. Some of them can also be used to jumpstart your car, if you run into trouble in a remote area. You may be able to plug a solar powered battery charger into them as well, solar cells generate power as long as enough light reaches them. If you need lots of power to run your telescope, then one, or even two deep cycle marine batteries are needed, and you will need an automobile battery charger to charge this kind of battery. Be careful, a pair of these batteries wired in series are powerful enough to TIG weld metal, so some sort of non-metallic container should be used to keep them from shorting or spilling the electrolyte. You should let someone know where you are going and carry a cell phone. Many very remote areas are beyond the reach of them, but without one help could be a very long time in coming if you break down in a very remote area. I would take along extra clothing and anything else you will need to deal with the winter weather too.

The Harbor Freight one in your link has a 17 Amp hour battery, the Wally World one is only 5 Amp hours - not nearly enough. I do, however, think that Wall-Mart might possibly have a more expensive unit with a higher Amp hour rating.

These suggestions of the rechargeable power tanks that need 110v to charge are not relevant . If we read the thread , the O.P. states , one week trip , remote location , only source of power is rental car . Hello !

Thank you for all the input and suggestions this far. I realize that I should have provided more detailed information about my trip.

1. I fly from home to the location where I will rent the car. I am trying to limit baggage to one 50 lb checked item plus carryon.

2. As Dave Starrancher correctly pointed out, I will not have access to 110 volt recharging facility. I am planning to camp out at my observation sites.

3. I have a limited budget for this trip. The airfare plus car rental was in the $500 range.

4. Normally a 17 Ah powerpack is sufficient for my power needs for one night. (But I am not taking the powerpack on the airplne due to weight.)

I have the following thought this far.

A. Once I saw at a truckstop store a cigarette lighter plug, that had some simple electronics built in. It monitored the battery voltage and prevented to deplete it to below whatever is needed to start the car. Something like this is good to have in general and is small and not expensive.

B. I need to find out more about charging a depleted battery in a car. My gut feeling is that it is not a good idea to connect a depleted battery unless the engine is running. I believe that this is where the insulator is coming into play. Apart from that, an idling engine may not be enough to recharge a battery. I need to do more homework on this.

C. I need to look into the small generators that one of you suggested. If it costs $100 and is small enough to ship it home by USPS after the trip, then it may be a justifiable expense. Unfortunately that would also need an extra battery. I don't believe that such a generator provides smooth enough voltage to use it to directly power astro equipment.

D. I am afraid of relying on previously untested equipment, like a solar charger. I simply don't know, if is capable of recharging a 18 Ah battery. In Seattle, where I live such a thing would not recharge a battery during the entire month of December... I have no way to test it before the trip.

Thanks again for all the suggestions this far. I am still open to more ideas.

And I would stay away from the 100 dollar generators..I believe most are two cycle, at least the ones I see for cheap...and are square wave..they can do a job on sensitive electronics...and once you load them up with fuel and run them once..no way they will probably ship thru any carrier.....

I suggested the Power Tanks because, they are safe for the uninitiated. Besides the Power Tanks, the next best option....probably as cheap-most efficient-expedient is the purchase of a 12 volt lawn mower battery from Wally World or the auto parts store, a fused 12 volt outlet from Radio Shack , an el cheapo digital volt meter and a set of jumper cables to charge said battery when you are not using it....

But then again, it is messy, as far as cables, monitoring the voltage on the battery, attaching the cables from battery to battery, etc. You need to learn just a wee bit of basic electronics so you don't run out of juice at an inopportune time and have a safe and enjoyable trip and don't blow yourself or one of the batteries up ..

I think it would be important to know if there will be other people there with you who will may have at a minimum, a car that can jump start yours if not a generator to charge you back up should you drain the battery at night. If there is support of this type, then just run your gear off the car battery.

If there is no other support, you need to separate the car and observing batteries. Given budget concerns, I'd buy the cheapest battery you can find that will run your equipment for as long as you need it to between times where you can run the car to charge it up. Don't buy the top-line "mega/gold/supreme" battery, get a generic car battery and keep it warm if possible through the night by insulating it - use a blanket you don't mind throwing away. Get some cheap jumper cables to charge it up off the rental car. An idling car doesn't produce nearly as much current as one revved up to 1500 RPM or so - no big deal, run the engine at an elevated speed for a time - if you need to.

Depending on the situation, running the car periodically can be a no-no as there will be a large and long-lasting heat plume coming off of it. If you must run it during your observation times, try to situate yourself and the car so that you don't try to look through the vertical space above the car. Make sure you don't spoil the views of others, if that's a concern.

If you are not using a GOTO mount and just tracking objects a car battery will last all night as long as it is not really cold out which you would be running the car to warm up in regularily anyway. I would disconnect the scope from the battery when starting the car tho. A solar charger, even on a cloudy day, can provide at least some charging and if you are camping, I would plug it in to the car once you are done observing for the night. If you are paranoid about killing the battery, running the car for 20 minutes or so every few hours will prevent it from discharging completely.

You might be able to rent a small Honda or other similar brand generator that outputs a sine wave and use that to run your scope. If I remember correctly, most of these also have 12V DC outputs. Worst case, you can use the car battery and then you'll have the generator to recharge it. Harbor Freight's 800W generator ($90) is a 2-cycle and requires a gas/oil mix. The more expensive Honda and others are 4-cycle units and use straight gasoline. You'll still need to bring a couple of gallons or so of gas in a can.

Car batteries are designed for short and large surges of electrical current. Period. They do not have sufficient amphour ratings to operate equipment for long. I've known of people who hook up multiple batteries to a car and have an external switch to charge both batteries. I'd guess that would be an option but it is kind of pointless in my opinion.

After you get off the plane, stop at any big box store driving away from the airport, buy a jumper battery, get a cheap one, it'll work for the time you need it.

Use the jumper battery to power your gear all night. The next day, start the car, then hook up the jumper battery as if you are trying to jump start the car. Let it recharge that way. Disconnect it to use it all night again.

On the way back to the airport, stop at a pawn shop, and retrieve a few bucks from it. They will only give you half of what it could be sold for, but it's a quick and easy way to retrieve a little of the price you paid for it.

this is the quick and easy, hassle free no shipping way to accomplish your goals.

The Duracell 600 has a 28ah AGM battery. Enough for a few nights use without question. I have 2 of them and use them 100+ days/nights a year and they are going on 4 years old now without a problem. When in the field, I charge them with a smart charger that I run from my car's 12v power outlet.